1. Field of the Invention
The present invention relates to a graphic arts apparatus for forming an image of an original by exposure on photosensitive material (such as a photosensitive film or a presensitized plate) in vacuum contact with the original, and a method therefor. In particular, the present invention relates to techniques for improving vacuum contact between an original film and photosensitive material.
2. Description of the Related Art
A known vacuum contact printing apparatus consists of a transparent plate, an elastic sheet in vacuum contact with the transparent plate, a squeegee roller which rolls on the back surface of the elastic sheet, and a vacuum pump for removing the air between the transparent plate and the elastic sheet. The squeegee roller is used to expel air from between the transparent plate and the elastic sheet by pressing the elastic sheet The purpose of removing the air from between the transparent plate and the elastic sheet is to make reliable contact between the original film and the photosensitive material placed thereon. The reliable contact enables a pattern on the original film to be printed faithfully and clearly on the photosensitive material.
A vacuum contact printing apparatus of interest to the present invention is disclosed in Japanese Utility Model Publication No. 56-5095. This apparatus was proposed by the assignee company of the present application. Referring to FIGS. 1 to 3, the conventional vacuum contact printing apparatus comprises: a table 4 for printing, having a lower frame provided with an elastic rubber sheet 1 with an air outlet 9, and an airtight seal 2 formed to surround the rubber sheet 1; an upper frame 7 which is openable and closable with respect to the table 4, and having an edge frame 5 and a transparent plate 6 provided on the lower surface of the edge frame 5; a squeegee roller 11 for accelerating removal of the air from the space defined by the transparent plate 6, the rubber sheet 1 and the airtight seal 2; and a squeegee roller drive unit 40 for driving the squeegee roller 11. This apparatus further comprises evacuation means, not shown, for removing the air through the air outlet 9.
The original film and a photosensitive material are placed on the rubber sheet 1 and a transparent plate 6 is placed thereon. To prevent the rubber sheet 1 from being deformed downwardly by the weight of the original film or the photosensitive film, the apparatus further comprises a rubber sheet support unit 41 for supporting the rubber sheet 1 from below when the original film and the photosensitive material are placed on the rubber sheet 1.
The rubber sheet support unit 41 comprises: a vertically movable support plate 16 which contacts the lower surface of the rubber sheet 1 in its uppermost position; a pair of guide plates 48 fixed to the support plate 16 in juxtaposed relationship with each other in a direction perpendicular to the plane of the drawing to extend downward from both ends of the support plate 16, and having respective front end portions which bend obliquely downward; a contact member 49 provided on the lower end of each guide plate 48; and compression coil springs 51 provided between the lower surface of a base frame 26 and each contact member 49, for biasing the corresponding guide plate 48 upward.
In the central portion of each guide plate 48 is a guide groove 50. The grooves 50 are formed with a bend similar to the form of the guide plate 48. The rubber sheet support unit 41 further includes a contact member, not shown, for stably setting the support plate 16 in alignment with the upper surface of the base frame 26.
The squeegee roller drive unit 40 comprises: a pair of guide rails 14 provided on the right and left sides of a lower portion of the base frame 26 in juxtaposed relationship with each other and extending in the direction of the base frame 26; sliding tables 53 which slide along the guide rails 14; roller support arms 56 provided on the respective sliding tables 53, for rotatably supporting the squeegee roller 11; a guide roller 58 which is rotatably supported by intermediate portions of the roller support arms 56, with both of its ends inserted in the guide grooves 50, so as to be coupled to the guide plates 48; a pair of sprockets 54 provided near the ends of the base frame 26, under each guide rail 14; an endless chain 55 on each pair of sprockets 54; and a motor, not shown, for rotating the sprockets 54 in synchronization. The lower end of each sliding table 53 is coupled to the corresponding endless chain 55. A triangular guide member 59 is provided on the lower surface of the front edge 57 of the base frame 26.
In operation, the squeegee roller 11 is first located at the foremost position in the base frame 26. Since the guide roller 58 is in the front ends of the guide grooves 50, the guide plates 48 are pushed upward by the springs 51. The support plate 16 supports the rubber sheet 1 from below.
The operator opens the upper frame 7 and places an original film 31 and a photosensitive material 32 on the rubber sheet 1. After closing the upper frame 7, the operator presses a start button at an operation panel (not shown). The evacuating means, not shown, removes air through the air outlet 9 from a space 8 defined by the transparent plate 6, the table 4 and the airtight seal 2.
While air is being removed from the space 8, the motor rotates the sprockets 5. This causes the chains 55 to rotate to cause the upper portions thereof to move toward the rear side of the base frame 26. Since the lower ends of the sliding tables 53 are coupled to the corresponding endless chains 55, the sliding tables 53 move on the corresponding guide rails 14 toward the rear end of the base frame 26. As the guide roller 58 moves together with the sliding tables 53, the higher portions of the guide grooves 50 contact the guide roller 58. Since the vertical position of the guide roller 58 is fixed, the guide plates 48 are pushed downward by the guide roller 58 as the sliding tables 53 move to the rear. The support plate 16 is thus pulled down away from the lower surface of the rubber sheet 1.
The squeegee roller 11 travels while pressing against the lower surface of the rubber sheet 1. The squeegee roller 11 forces the air in the space 8 toward the air outlet 9 and the air is discharged through the air outlet 9. Thus, the blowhole often left between the original film 31 or the photosensitive material 32 and the rubber sheet 1 can be removed by the pressing force of the squeegee roller 11.
When the air is sufficiently removed, exposure is carried out. The squeegee roller 11 is then returned to the initial position according to procedures opposite to those before the exposure. In this case, however, the squeegee roller 11 does not necessarily need to press the lower surface of the rubber sheet 1. When the squeegee roller 11 is returned to the initial position, the guide plates 48 and the support plate 16 are pushed upward by the upward biasing force of the springs 51, to support the lower surface of the rubber sheet 1.
Another rubber sheet support unit 44 (FIG. 4) comprises: a rectangular support plate 60; vertical guide shafts 61 provided under the corners of the support plate 60; bearinqs 62 through which the corresponding guide shafts 61 are slidably engaged; a rod 64 which is rotatably coupled to a bearing 63; a crank 65 which is rotatably coupled to the rod 64; and a drive unit, not shown, for rotating the crank 65. The bearing 63 is located under the center of the support plate 60.
When the crank 65 is rotated by the drive unit, the rod 64 moves downward. Since the support plate 60 is coupled to the rod 64 through the bearing 63, it moves downward. The bearings 62 slidably support the guide shafts 61 so that the support plate 60 can be maintained in a horizontal position. When the crank 65 is rotated in the reverse direction by the drive unit, the support plate 60 can be raised while being maintained in a horizontal position.
In the above described conventional apparatus, the use of the squeegee roller 11 makes it possible to reduce the time required for achieving a vacuum contact. However, to further improve the function of such an apparatus, it is necessary to solve the following problems.
One of the problems is that the time required for completing vacuum contact in the conventional apparatus could be further reduced. Referring to FIG. 5A, at an initial stage, peripheral portions of the rubber sheet 1 contact the transparent plate before the central portion of the rubber sheet. This is because the air outlet 9 is provided in a peripheral portion of the rubber sheet 1 and the pressure in peripheral portions of the airtight space 8 is reduced first. The original film 31 and the photosensitive material 32 are normally placed in the central portion of the rubber sheet 1. Accordingly, a blowhole 33 is liable to be formed between the original film 31 and the photosensitive material 32. The squeegee roller 11 advances while pressing on the blowhole 33.
Referring to FIG. 5B, the squeegee roller 11, while travelling, squeezes the blowhole 33 toward the rear edge of the original film 31. However, as described above, vacuum contact has already been made around the original film 31 and the photosensitive material 32. Consequently, the air in the blowhole 33 cannot be easily moved to the peripheral portions. Nevertheless, the squeegee roller 11 continues to move.
FIG. 5C illustrates the partially dissolved blowhole 33 of the air which cannot follow the movement of the squeegee roller 11. The squeegee roller 11 moves over and past the blowhole 33 before the air in the blowhole 33 is completely moved.
Clear exposure cannot be carried out while the blowhole 33 shown in FIG. 5C exists. Therefore, it is necessary to continue the removal of the air. The squeegee roller 11 no longer serves to promote the removal of air. To sufficiently remove air, more time is required.
Another problem is that it is not so easy in the conventional apparatus to place the original film 31 and the photosensitive material 32 accurately on the rubber sheet 1.
In the apparatus shown in FIGS. 2 and 3, the support plate 16 is supported by the guide plates 48, which are forced upward by the springs 51. When the support plate 16 contacts the lower surface of the rubber sheet 1, the extent to which the springs 51 are compressed is relatively small. If force is applied to the rubber sheet 1 from above, it is transmitted to the springs 51 through the support plate 16 and the guide plates 48. The springs 51 are compressed with relative ease. As a result, the rubber sheet 1 can be moved vertically by a relatively small force. Consequently, it is difficult to place the original film 31 and the photosensitive material 30 on the rubber sheet 1 with stability.
High precision is required for positioning in an exposure process in contact printing. To accurately position the film 31 and the material 30, it is sometimes necessary to place one's hand on the rubber sheet 1. When the photosensitive material 30 is to be taped to the rubber sheet 1, or when the original film 31 is to be taped to the photosensitive material 30 after the material 30 is placed on the rubber sheet 1, it is necessary to apply prescribed pressure to the rubber sheet 1. Since the support plate 16 easily fluctuates vertically, it is not easy to carry out the above mentioned work.
The parts of the system shown in FIG. 4 must be manufactured and assembled with high precision. This increases the cost of manufacturing. Furthermore, the support plate 60 must be stiffer than necessary to support the rubber sheet 1, increasing the cost of manufacturing the entire apparatus. Moreover, the apparatus has the below described structural disadvantages.
Most of the load applied to the rubber sheet 1 is applied to the crank 65 through the support plate 60 and the rod 64. The support plate 60 is raised or lowered by rotating the crank 65. Since the operator may put his hand on the rubber sheet 1 as described above, the drive system must be very durable. The drive unit must also be capable of operating under a high load. These facts result in increased size, lack of durability and excess production costs.